In a flap-gate breakwater of the prior art, the door body was raised or lowered by buoyancy (e.g., Patent Reference 1).
The prior art flap-gate breakwater raised the door body by supplying air to a buoyancy chamber provided to the door body, thereby discharging sea water from the buoyancy chamber, so it needed an air supply device to supply air to the buoyancy chamber and discharge water from the buoyancy chamber.
However, in order to be able to supply compressed air during an electricity breakout, the prior art flap-gate breakwater constantly required a reservoir of compressed air in an accumulator tank. It was also necessary to constantly monitor the pressure of the accumulator tank, the overturning moment of the door body (weight of the end of the door body), and the angle of inclination of the door body, because the buoyancy chamber of the door body fills with sea water when it is being contained, and the door body rests in the containment position because of its weight. However, in this case, it is impossible to detect abnormalities such as the formation of holes in the buoyancy chamber due to corrosion or the like. Moreover, if the weight of the door body increases due to sediments or the like, maintenance becomes burdensome, since it is necessary to maintain buoyancy operations or dredging.
If a tsunami warning is sounded during an earthquake and a breakwater is raised, it can take a long time to raise the breakwater, because an air supply valve is opened to supply air to the buoyancy chamber and water is discharged from the buoyancy chamber based on levitation instructions, and this can occur too late to block the influx of a tsunami.
Accordingly, the applicants had previously disclosed a mooring device which constantly held a door body in a buoyant state in a flap-gate breakwater in which the door body rises due to buoyancy (Japanese Patent Application No. 2008-307699).
In this prior art mooring device for a flap-gate breakwater, an operation of a mooring hook positioned in the water was carried out, using a mooring rope operated from land. Therefore, in order to maintain secure mooring conditions, the position of the mooring hook had to be adjusted if the mooring rope was stretched due to the passage of time or by seasonal changes, and such an adjustment takes a long time.
In addition, if the mooring rope stretched and needed to be replaced, the operation of attaching the end of the rope to the mooring hook had to be carried out under water.    Patent Reference 1: Japanese Patent Application Kokai Publication No. 2003-227125